The overall objective of the experiments proposed in this application is to enhance and validate a radionuclide imaging technique for evaluation of the origin and activation sequence of ventricular contraction during ventricular pacing or ventricular tachycardia (VT). The imaging technique (phase analysis) utilizes computer processing of radionuclide ventriculograms. The ultimate goal of this work is to determine the usefullness of phase imaging in defining the origin of VT, since if accurate it may guide surgical excision of the abnormal tissue responsible for the VT. We will examine in detail the correlation between phase imaging and electrical activation sequence determined by endocardial and epicardial mapping of the ventricles. We will evaluate phase imaging in both animal models and in humans who are undergoing surgical treatment of VT. Our specific goals are 1) to improve the accuracy of phase imaging by exploring alternative data acquisition and processing techniques; 2) to determine if global or regional left ventricular dysfunction limits the accuracy of phase imaging; 3) to measure the accuracy of phase imaging for determining the origin and sequence of electrical activation during VT in both dog and man. Investigations will be performed in chronic dogs with implanted pacemakers, dogs with wall motion abnormalities, dogs with VT following myocardial infarction, and in patients with recurrent VT. In the dog model, both endocardial and epicardial mapping of the electrical activation sequence will be performed. In humans, phase imaging will be performed during preoperative endocardial mapping in the catheterization laboratory and epicardial mapping will be performed intra-operatively. A major strength of our proposal is our ability to compare the data derived from radionuclide imaging to electrophysiological measurements of the actual activation sequence in the ventricles. The major aspects of the proposed experiments have been demonstrated to be feasible in our preliminary studies. Our ability to reach our goals is enhanced by a research environment fostering effective collaboration between radiologists, cardiologists, computer engineers, and cardiovascular surgeons.